Nitromethane which was used during the late 1940's as a rocket monopropellant, has been used in racing as a fuel for over 30 years. The use of fuels which carry a bonded oxygen in their chemical formula is sometimes referred to as "thermal charging" and is a quick and relatively inexpensive method for increasing the available energy for combustion, provided that the heating value of the fuel does not decrease more than the decrease in the air-to-fuel ratio, as compared with fuels which contain no oxygen.
Although the use of nitromethane significantly increases the horsepower of an internal combustion engine, when compared, for example, with the same engine using a classical gasoline mixture, the tendency of nitromethane to detonate creates a substantial problem for its use. Uncontrolled detonation in an internal combustion engine can, of course, lead to its destruction.
In order to understand the problem of detonation, one must first appreciate that in an internal combustion engine, the flame which is initiated by the spark travels across the combustion chamber at a given velocity, usually about 200 feet per second. As the flame front progresses across the combustion chamber, the remaining gases, termed end gasses, are compressed by the rising pressure, resulting in higher gas temperatures and densities. If these gas temperatures exceed the self-ignition temperature of the air-gas mixture and, if the gas remains at such a temperature longer than its ignition-delay period, auto-ignition will occur. The auto-ignition, or detonation, can then generate pressure waves which may be an order of magnitude as fast as those in normal combustion.
Because nitromethane has a lower auto-ignition temperature than gasoline, detonation becomes the limiting factor in its use as a fuel for an internal combustion engine. Detonation, even when not severe, results in the engine audibly knocking which in itself is distracting. However, besides the audible noises generated by the detonation, the pressure waves which are induced by the detonation penetrate what typically exist in the form of a boundary layers along the cylinder walls which generally insulate the combustion chamber from the hot combustion gasses. As the boundary layer is scrubbed away, the heat losses from combustion products to the engine are increased, thereby reducing the useful energy in the cylinder. Also, repeated detonation may generate a hot region in the combustion chamber, as at the spark plug electrode, causing pre-ignition. When pre-ignition occurs, even higher temperatures and pressures are encountered in the combustion chamber, thus further aggravating the detonation problem. Thus, detonation and pre-ignition can become self-perpetuating.
As stated previously, severe detonation over a sustained period of time can substantially damage engine parts, such as pistons, valves, rings, and cylinder heads.
As auto-ignition is caused by explosion of the end gas, conditions which promote the explosion of the end gas, such as higher temperatures, increased density, increased exposure time of the unburned mixture, and the mixture composition itself, aggravate the problem.
Thus, although nitromethane offers the potential of substantially greater power than gasoline, when burned in an internal combustion engine, the problems associated with the tendency of nitromethane to detonate detracts substantially from its use as a fuel. To date, the only solution to the detonation problem has been the addition of methanol to the nitromethane fuel which acts to "cool" the burn of the nitromethane and thus to decrease its propensity to detonate. The problem with the addition of methanol, however, is that although the tendency to detonate is reduced, the addition of the methanol also substantially decreases the amount of energy which is obtained from the fuel formulation.
There thus exists a need for an additive for nitromethane which would decrease its tendency to detonate when burned in an internal combustion engine, while preserving, to the extent possible, the high specific energy of nitromethane.